Types of asexual reproduction ( Binary fission , Budding , Regeneration , Sporogony , Parthenogenesis & Tissues culture )
This type is common in the most primitive unicellular organisms such as Simple algae , Bacteria , many protozoans such as Amoeba and Paramecium , Binary fission happens under different conditions , In suitable conditions , The nucleus divides by mitosis , The cell ( that represents the unicellular organism ) divides into 2 cells , where each one of them becomes a new individual .
In unsuitable conditions , Amoeba ( unicellular organism ) secretes a chitinous coat around its body for protection , It divides within that coat several times by repeated binary fission to produce many young Amoeba , The young Amoeba are released from the cyst once the surrounding conditions are improved .
In unicellular organisms
- The bud arises as a lateral projection on the original cell .
- The nucleus divides mitotically into 2 nuclei , where one of them remains in the parent cell , while the other moves towards the bud .
- The bud grows gradually , then it may remain connected with the parent cell till its full growth , then it separates from it or continue in its connection with the parent cell , forming cellular colonies with the other growing buds .
Example : Yeast fungus
In multicellular organisms
- The bud grows as a small cellular protrision from one side of the body , due to the division of the interstitial cells and their differentiation to a bud .
- The bud grows gradually to resemble the parent entirely .
- The new individual usually separates to start its life independantly .
Examples : Hydra , Sponges .
This method is common in many plants , Some animals such as sponges and Hydra and sea star ( starfish ) , Some worms such as Planaria , The capacity for regeneration decreases by increasing the animals evolution .
Higher animals ( regeneration for compensation )
Regeneration is not considered as a reproduction in some organisms , where it is limited on compensating the lost parts of their body , due to an accident or a rupture of the tissues .
- Higher vertebrates : regeneration never exceeds the healing of wounds , especially those located in the skin , blood vessels and muscles .
- Some crustaceans and amphibians : regeneration is limited on the compensation of the cut parts only .
Lower animals ( regeneration for reproduction )
Regeneration is considered as a reproduction in some organsisms , as any cut part will grow into a new individual .
- Hydra is able to regenerate , as if it is cut into several transverse pieces , where each part will grow into a new individual .
- Planaria ( common fresh water flat worm ) is able to regenerate , even if it is cut into several transverse pieces or 2 longitudinal parts , where each part will grow into a new individual .
- Sea star ( Starfish ) can regenerate , if one of its arms is cut with a piece of its central disc to a full sea star within a short time , this type of sea stars forms a danger on the pearl mussels since the one sea star can devour about ten mussels daily including the pearl , therfore the breeders of those mussels in the pearl farms were collecting the sea stars , tearing them up into pieces , and then throw them back into the sea , Although they tried to get rid of them , they were unintentionally helping in their reproduction, so that they resorted to burn them .
Some primitive plants reproduce by means of a single cells called spores that are modified to grow directly into a complete plant .
A spore is formed of a cytoplasm with a little amount of water , a nucleus and surrounded by a thick coat , It is a single cell that modified to grow directly into a new complete organism under suitable conditions for its growth such as in some primitive plants .
Sporogony stages :
- After the spore maturation , it liberates from the mother plant to distribute in the air .
- When it reaches a suitable medium for growth , it absorbs water and its coat ruptures .
- It divides several times by mitosis , till it grows into a new individual .
Examples : Many fungi such as bread mould and mushroom , Some algae and ferns .
Advantages of sporogony :
- Quick propagation .
- Tolerance to hard conditions .
- Distribution to distant regions .
Parthenogenesis is the ability of the ovum to develop into a new individual without being fertilized by a male gamete , Parthenogenesis occurs in a number of worms , crustaceans and insects .
Parthenogenesis is considered as a special kind of asexual reproduction , where the progeny comes only from one parent only which is the mother’s gamete , Parthenogenesis can happen naturally or artificially as follows .
Honey bee : the queen produces the eggs by meiotic division , where :
- Some eggs grow by parthenogenesis and develop without fertilization ( asexual reproduction ) , forming drones that are haploid ( n ) .
- Some others develop after fertilization ( sexual reproduction ) to form the queen and workers that are diploid ( 2n ) .
Aphid insect : the eggs result from a mitotic division ( without fertilization ) , where they develop into diploid ( 2n ) individuals .
Frog and sea star : Parthenogenesis has been induced artificially by activating the eggs through their exposure to thermal or electric shock or radiation some salt or agitation or pricking with a needle , These stimuli lead to the duplication of the chromosomes without fertilization , forming new individuals that are totally identical to their mother .
Rabbits : by using stimulants that are similar to the previous stimuli to form early embryonic stages of rabbits from their ova .
Scientists carry out experiments on the animal and plant tissues culture , Tissues culture is the growth of a living tissue ( its cells contain the whole genetic information ) in a proper semi-natural nutritive medium and following its tissues differentiation and progress into a full developed organism .
Experiment ( 1 ) on carrot plant :
Separation of small pieces of carrot plant in glass tubes containing the coconut milk ( which comprises the whole plant hormones and nutrient elements that are necessary for plant growth ) , then these pieces began to grow and differentiate into a full carrot plant .
Separation of some cells from the same plant tissues and cultivated them by the same method to obtain a whole plant .
Experiment ( 2 ) on tobacco plant :
Separation of some cells from tobacco leaves and cultivating them by the same previous method , so that we can obtain a full tobacco plant .
The scientific idea for plant tissues culture : Any somatic cell in the plant that comprises the whole genetic information can develop into a full plant , if it is cultivated in a proper nutritive medium , containing the plant hormones with a certain ratio .
The importance of tissue culture
- Propagation of rare plants or those with desirable strains or more resistant to the diseases .
- Shortening the time needed for growing and increasing the crops .
- Solving the food problems in general .
- Controlling the time of tissue culture , where the selected tissues for such culture can be preserved in a liquid nitrogen for a long period to keep them alive and maintain their vatility till their cultivation .